How can I fit Clothes to a Body?
I don't know if there's an easy way to do it, I would do it manually, but you could try to do it with a Mesh Deform modifier (I tried with an armature then apply it but it was not as good):


Create a low-poly cage around the object you want to deform. In the Properties panel > Object > Display > Maximum Draw Type, choose Wire to make it transparent.





Give the high-poly a Mesh Deform modifier with the low-poly as Object, click on the Bind button.





Deform the low-poly cage.





When you're glad, select the high-poly and click on the Apply button.
Make some corrections in Edit mode with the Proportional Editing mode enabled.